As Alexei noted, get_perf_callchain() return values may be reused if a task is preempted after the BPF program enters migrate disable mode. We therefore use bpf_perf_callchain_entries percpu entries similarly to bpf_try_get_buffers to preserve the current task's callchain and prevent overwriting by preempting tasks. And we also add preempt_disable to protect get_perf_callchain. Reported-by: Alexei Starovoitov Closes: https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com Signed-off-by: Tao Chen --- kernel/bpf/stackmap.c | 76 ++++++++++++++++++++++++++++++++++--------- 1 file changed, 61 insertions(+), 15 deletions(-) Change list: v1 -> v2: From Alexei - create percpu entris to preserve current task's callchain similarly to bpf_try_get_buffers. v1: https://lore.kernel.org/bpf/20250922075333.1452803-1-chen.dylane@linux.dev diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c index 2e182a3ac4c..8788c219926 100644 --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -31,6 +31,55 @@ struct bpf_stack_map { struct stack_map_bucket *buckets[] __counted_by(n_buckets); }; +struct bpf_perf_callchain_entry { + u64 nr; + u64 ip[PERF_MAX_STACK_DEPTH]; +}; + +#define MAX_PERF_CALLCHAIN_PREEMPT 3 +static DEFINE_PER_CPU(struct bpf_perf_callchain_entry[MAX_PERF_CALLCHAIN_PREEMPT], + bpf_perf_callchain_entries); +static DEFINE_PER_CPU(int, bpf_perf_callchain_preempt_cnt); + +static int bpf_get_perf_callchain(struct bpf_perf_callchain_entry **entry, + struct pt_regs *regs, u32 init_nr, bool kernel, + bool user, u32 max_stack, bool crosstack, + bool add_mark) +{ + struct bpf_perf_callchain_entry *bpf_entry; + struct perf_callchain_entry *perf_entry; + int preempt_cnt; + + preempt_cnt = this_cpu_inc_return(bpf_perf_callchain_preempt_cnt); + if (WARN_ON_ONCE(preempt_cnt > MAX_PERF_CALLCHAIN_PREEMPT)) { + this_cpu_dec(bpf_perf_callchain_preempt_cnt); + return -EBUSY; + } + + bpf_entry = this_cpu_ptr(&bpf_perf_callchain_entries[preempt_cnt - 1]); + + preempt_disable(); + perf_entry = get_perf_callchain(regs, init_nr, kernel, user, max_stack, + crosstack, add_mark); + if (unlikely(!perf_entry)) { + preempt_enable(); + this_cpu_dec(bpf_perf_callchain_preempt_cnt); + return -EFAULT; + } + memcpy(bpf_entry, perf_entry, sizeof(u64) * (perf_entry->nr + 1)); + *entry = bpf_entry; + preempt_enable(); + + return 0; +} + +static void bpf_put_perf_callchain(void) +{ + if (WARN_ON_ONCE(this_cpu_read(bpf_perf_callchain_preempt_cnt) == 0)) + return; + this_cpu_dec(bpf_perf_callchain_preempt_cnt); +} + static inline bool stack_map_use_build_id(struct bpf_map *map) { return (map->map_flags & BPF_F_STACK_BUILD_ID); @@ -303,8 +352,9 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, u32 max_depth = map->value_size / stack_map_data_size(map); u32 skip = flags & BPF_F_SKIP_FIELD_MASK; bool user = flags & BPF_F_USER_STACK; - struct perf_callchain_entry *trace; + struct bpf_perf_callchain_entry *trace; bool kernel = !user; + int err; if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID))) @@ -314,14 +364,15 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, if (max_depth > sysctl_perf_event_max_stack) max_depth = sysctl_perf_event_max_stack; - trace = get_perf_callchain(regs, 0, kernel, user, max_depth, - false, false); + err = bpf_get_perf_callchain(&trace, regs, 0, kernel, user, max_depth, + false, false); + if (err) + return err; - if (unlikely(!trace)) - /* couldn't fetch the stack trace */ - return -EFAULT; + err = __bpf_get_stackid(map, (struct perf_callchain_entry *)trace, flags); + bpf_put_perf_callchain(); - return __bpf_get_stackid(map, trace, flags); + return err; } const struct bpf_func_proto bpf_get_stackid_proto = { @@ -443,8 +494,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, if (sysctl_perf_event_max_stack < max_depth) max_depth = sysctl_perf_event_max_stack; - if (may_fault) - rcu_read_lock(); /* need RCU for perf's callchain below */ + preempt_disable(); if (trace_in) trace = trace_in; @@ -455,8 +505,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, crosstask, false); if (unlikely(!trace) || trace->nr < skip) { - if (may_fault) - rcu_read_unlock(); + preempt_enable(); goto err_fault; } @@ -474,10 +523,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, } else { memcpy(buf, ips, copy_len); } - - /* trace/ips should not be dereferenced after this point */ - if (may_fault) - rcu_read_unlock(); + preempt_enable(); if (user_build_id) stack_map_get_build_id_offset(buf, trace_nr, user, may_fault); -- 2.48.1